The present invention relates to a scanning optical system to be installed in printing devices such as color printers and color copy machines.
As is widely known, a scanning optical system dynamically deflects a laser beam (which has been on-off modulated according to image information) by use of a rotational polygon mirror and converges the dynamically deflected laser beam on a surface of a photosensitive drum (scan target surface) by use of an imaging optical system, by which the spot beam is scanned on the scan target surface at a constant speed in a main scanning direction and thereby an electrostatic latent image composed of a plurality of dots is formed on the scan target surface.
In conventional scanning optical systems installed in color printing devices, each photosensitive drum for each color component (e.g. yellow, magenta, cyan, black) was generally provided with one rotational polygon mirror and one imaging optical system for its own (see U.S. Pat. No. 6,452,687, for example) However, there have recently been developed scanning optical systems in which a rotational polygon mirror and part of an imaging optical system are shared among the color components in order to reduce manufacturing costs of printing devices (see U.S. Pat. No. 6,313,906, for example).
In such a scanning optical system, each of N laser beams (N: the number of color components) before reaching the rotational polygon mirror may be xe2x80x9ctiltedxe2x80x9d in a plane that is parallel to the central axis of the polygon mirror so that the beams will travel along the plane and then intersect at a point in the vicinity of a reflecting surface of the polygon mirror. Such a technique has been elaborated on in the specification and figures of U.S. patent application Ser. No. 10/294,615 previously filed by the assignee of the present invention. In this case, after being reflected by the rotational polygon mirror, the N laser beams pass through a common front lens group of the imaging optical system gradually separating from one another as shown in FIG. 9 (cross-sectional view of the scanning optical system), pass through N rear lens groups of the imaging optical system respectively, and are then incident on N photosensitive drums parallelly arranged at preset intervals respectively.
Incidentally, while the photosensitive drums shown in FIG. 9 overlap with one another, enough space is kept between any two adjacent photosensitive drums in actual scanning optical systems so that necessary parts (development module, toner cartridge, etc.) can be placed in the space. Therefore, in such a scanning optical system, any two adjacent laser beams to be incident on the photosensitive drums have to be separated from each other to match the distance (interval) between adjacent photosensitive drums.
In order to realize the separation of the laser beams, the scanning optical system has to be designed employing one of the following three configurations: a first configuration in which the optical path length between the polygon mirror and each photosensitive drum is set long; a second configuration in which the angle between adjacent laser beams incident on the polygon mirror is set large; and a third configuration in which the optical path of each laser beam after passing through the front lens group is bent by one or more mirrors and thereby the separation between adjacent laser beams is increased as shown in FIG. 10.
However, the first configuration has the disadvantage of increasing the size of the printing device. Meanwhile, in the second configuration, it is difficult to correct scan line curves and twist of wavefront aberration (twist of the wavefront) at the same time, by which attaining enough scanning performance becomes difficult. Consequently, the third configuration like the one shown in FIG. 10 is generally employed for the above-described scanning optical system.
However, in the third configuration like FIG. 10 in which mirrors are placed on the optical paths of the laser beams, the cost for manufacturing the system rises as the number of optical elements increases. Further, if the precision of each mirror surface Is not maintained high, scanning performance of the system tends to vary among the color components and thereby misregistration or misalignment in color superposition (hereafter called xe2x80x9ccolor misregistrationxe2x80x9d) occurs when a plurality of images of the color components are overlaid.
The present invention is advantageous in that it provides a scanning optical system capable of sufficiently separating the laser beams to match the intervals between the photosensitive drums without the need of long optical paths, without deteriorating the scanning performance, and without the need of using mirrors causing the color misregistration and increasing the cost.
In accordance with an aspect of the present invention, there is provided a scanning optical system for dynamically deflecting a plurality of laser beams by a deflecting system for a plurality of scan targets which are provided corresponding to the laser beams and parallelly arranged at preset intervals, converging the dynamically deflected laser b ams by an imaging optical system into spot beams on the corresponding scan targets respectively, and thereby scanning the spot beams in a main scanning direction on the corresponding scan targets respectively. In the scanning optical system, the deflecting system includes at least one reflecting surface which simultaneously deflects the laser beams that are incident on the reflecting surface at incident angles differing in an auxiliary scanning direction perpendicular to the main scanning direction. The imaging optical system includes: a front lens group having positive refractive power for converging all the laser beams from the deflecting system principally in the main scanning direction while deflecting at least a pair of laser beams selected from the laser beams obliquely incident on the reflecting surface so as to let the selected laser beams deviate from an optical surface reference axis of the front lens group; and a plurality of rear lens groups each of which has positive refractive power for converging each of the laser beams that passed through the front lens group principally in the auxiliary scanning direction.
With such configuration of the scanning optical system, even when the optical path length is relatively short, scan line curves can be satisfactorily corrected while fulfilling basic specs of scanning performance required of scanning optical systems, and furthermore, the twist of the wavefront can be reduced to a sufficiently low level. Therefore, the laser beams can be separated sufficiently to match the intervals between the scan targets without the need of using mirrors which cause the color misregistration and increasing the cost, by which variations in the scanning performance causing the color misregistration can be prevented.
Optionally, at least one surface of the front lens group may have a plurality of areas of different shapes for interacting with the laser beams respectively.
Still optionally, the at least one surface of the front lens group may be formed as a step-like optical surface in which lens thickness changes at each boundary between adjacent areas.
Still optionally, each area of the at least one surface of the front lens group may be formed as a two-dimensional polynomial aspherical surface which is expressed by a polynomial expression regarding heights in the main scanning direction and the auxiliary scanning direction.
In a particular case, the two-dimensional polynomial aspherical surface of the front lens group may be asymmetric in the main scanning direction.
Additionally or alternatively, the two-dimensional polynomial aspherical surface of the front lens group may be asymmetric in the auxiliary scanning direction.
Optionally, surfaces on at least one side of the rear lens groups may be formed so as not to be in the same shape.
Still optionally, at least one surface of each of the rear lens groups may be formed as a two-dimensional polynomial aspherical surface which is expressed by a polynomial expression regarding heights in the main scanning direction and the auxiliary scanning direction.
In a particular case, the two-dimensional polynomial aspherical surface of each of the rear lens groups may be symmetric with respect to the optical surface reference axis in the main scanning direction.
Additionally or alternatively, the two-dimensional polynomial aspherical surface of each of the rear lens groups may be asymmetric in the auxiliary scanning direction.
Optionally, the pair of laser beams obliquely incident on the deflecting system travel at the same tilt angle on both sides of a main scanning plane which is defined as an imaginary plane parallel to the main scanning direction and including the optical surface reference axis of the front lens group. In this case, rear lens groups for transmitting the pair of laser beams respectively are formed in shapes mirror-symmetrical with each other with respect to the main scanning plane as a symmetry plane.
Still optionally, the pair of laser beams obliquely incident on the deflecting system travel at the same tilt angle on both sides of a main scanning plane which is defined as an imaginary plane parallel to the main scanning direction and including the optical surface reference axis of the front lens group. In this case, areas of the front lens group for interacting with the pair of laser beams respectively are formed in shapes mirror-symmetrical with each other with respect to the main scanning plane as a symmetry plane.
Still optionally, the pair of beams are deviated from the optical surface reference axis of the front lens group by the front lens group in the auxiliary scanning direction.
According to another aspect of the invention, there is provided a scanning optical system for dynamically deflecting a plurality of laser beams by a deflecting system for a plurality of scan targets which are provided corresponding to the laser beams and parallelly arranged at preset intervals, converging the dynamically deflected laser beams by an imaging optical system into spot beams on said corresponding scan targets respectively, and thereby scanning the spot beams in a main scanning direction on said corresponding scan targets respectively. In the scanning optical system, the deflecting system includes at least one reflecting surface which simultaneously deflects the laser beams that are incident on the at least one reflecting surface at incident angles differing in an auxiliary scanning direction perpendicular to the main scanning direction. The imaging optical system includes: a front lens group having positive refractive power for converging all the laser beams from said deflecting system principally in the main scanning direction while deflecting at least a pair of laser beams selected from the laser beams so that the pair of laser beams are deviated from an optical surface reference axis of said front lens group in the auxiliary scanning direction; and a plurality of rear lens groups each of which has positive refractive power for converging each of the laser beams that passed through said front lens group principally in the auxiliary scanning direction.
With such configuration of the scanning optical system, even when the optical path length is relatively short, scan line curves can be satisfactorily corrected while fulfilling basic specs of scanning performance required of scanning optical systems, and furthermore, the twist of the wavefront can be reduced to a sufficiently low level. Therefore, the laser beams can be separated sufficiently to match the intervals between the scan targets without the need of using mirrors which cause the color misregistration and increasing the cost, by which variations in the scanning performance causing the color misregistration can be prevented.